Branched fatty acids exist in nature (http://www.cyberlipid.org/cyberlip/home0001.htm), but are extremely rare, sparingly commercially available and particularly expensive. To illustrate the rarity of these products, it may be noted that the price of products of this category is much higher than that of the unbranched products, for example in the catalog from the Aldrich company http://www.sigmaaldrich.com/catalog/product/sigma/m6281?lang=fr&region=FR.
Various processes for synthesizing these branched unsaturated fatty acids are described in the literature. Mention may be made of the isomerization of fatty acids under heterogeneous catalysis; however, this process leads to a broad distribution of various branched fatty acids. Mention may be made in this respect of the article by U. Biermann et al. in Eur. J. Lipid Sci. Technology, entitled Synthesis of alkyl branched fatty acids 2008, 110, 805-811 and that of Zhang et al., published in The proceedings of the 3rd International Conference on Functional Molecules, entitled Novel Process to Produce Branched Fatty Acid/Ester for Biodiesel and New Surfactant. 
Various other methods such as the alkylation of olefins, hydroformylation and hydrogenation processes, fatty acid dimerization processes, Friedel-Crafts reactions, reactions involving formaldehyde (ENE reaction) and radical additions have been studied, as evidenced by the article from Ivan Malchev entitled Plant Oil based Lubricants available from the Department of Plant Agriculture, Ontario Agriculture College, University of Guelph, 50 Stone Road W., Guelph, Ontario, Canada N1G 2W1 and downloaded from the Internet at http://ebookbrowse.com/i-malchev-pdf-d92064738 and http://www.pdfio.com/k-1103149.html. However, it should be noted that, globally, none of these methods is entirely satisfactory. The products obtained by catalytic isomerization are mixtures of isomers; the products obtained by hydroformylation are saturated and certain cold properties are thus lost; the addition products, especially from radical addition, are mixtures, and it is not possible to obtain a quite specific product.
Another recent approach, described especially in patent EP 1 019 512, consists in identifying the genes in plants that lead to branched acids and in transferring them into microorganisms to make them produce specific acids. However, this method is merely in its infancy.
Branched compounds are distinguished from linear compounds by lower melting points and this property offers certain industrial openings. Mention may be made in this respect of isostearic acid, which is a commercial product, obtained as a by-product of the process for dimerizing oleic acid by catalysis with montmorillonite. The influence of isostearic acid on the spreading, viscosity and oxidation-stability and hydrolysis-stability properties of the products finds applications in the fields of cosmetics and lubrication (see the article by Biermann et al. cited previously).
In general, the strategy adopted by industrialists in the oleochemistry sector who have no access to branched compounds in sufficient concentration is to use as an alternative unsaturated compounds, which, themselves also, offer lower melting points than those of the corresponding saturated linear compounds. However, the unsaturated compounds are also less stable, and are sensitive to oxidation.
There is thus a real need to develop a quick and simple process (comprising the fewest possible steps) for synthesizing branched fatty compounds, especially branched unsaturated fatty acids.
In a known manner, cross metathesis consists in reacting, in the presence of a metathesis catalyst, two unsaturated molecules according to the following schematic reaction process:A1A2−C≡C−B1B2+D1D2−C≡C−E1E2A1A2−C≡C−D1D2+A1A2−C≡C−E1E2+B1B2−C≡C−D1D2+B1B2−C≡C−E1E2 
(cross metathesis), andA1A2−C≡C−A1A2+B1B2−C≡C−B1B2+D1D2−C≡C−D1D2+E1E2−C≡C−E1E2 
(homometathesis)
Cross metathesis is used for the purpose of synthesizing a target product, for example A1A2−C≡C−D1D2 in the above equation, but which will be accompanied, where appropriate, by the other three possible compounds derived from the cross metathesis of the same two reagents, and also certain compounds derived from the homometathesis of each reagent, this reaction accompanying the main reaction.
The use of metathesis was described by the company Elevance for the manufacture of fuel bases, especially aircraft fuels, from plant oils. Thus, patents US 2011/0113679 and US 2011/0237850 describe the reaction between a natural plant oil and a mixture of light olefins, leading, after additional treatment, to a mixture of compounds in which the number of carbon atoms is variable, multiple and uncontrolled, and whose properties in terms of energy density and flash point are suitable for the manufacture of fuels. Nevertheless, this type of mixture is not suitable for applications that are more stringent especially in terms of purity. Furthermore, the process described in each of these documents does not make it possible economically to manufacture compounds with a low melting point, i.e. with a melting point (m.p.) below 0° C., preferably below −20° C., even more preferably below −30° C.
The present invention is directed, on the contrary, toward providing a process in which the selectivity, the purity and thus the physicochemical properties of the branched unsaturated fatty compound are controlled, to be able to use it in high-performance technical applications, such as dielectric fluids, specialty surfactants, emulsifiers, friction agents, antistatic additives, antifogging additives, mold-release agents, pigment dispersants, high-performance lubricants, waxes and wax emulsifiers, polymer transformation additives, PVC stabilizers, inks, resins, paints, varnishes, solvents, lipsticks, skin creams, deodorants, especially in stick form, hair dyes, shampoos and other liquid soaps, shaving foams, detergents, cleaning agents, textile softeners, etc.
The Applicant has now found a means for making branched unsaturated compounds readily available, via a process using, under certain conditions, a cross metathesis reaction between at least two reagents selected according to the invention and making it possible more especially to target the synthesis of branched unsaturated fatty acid, ester or nitrile compounds. Unexpectedly, the process according to the invention promotes the synthesis of the target product, i.e. the synthesis of a branched unsaturated fatty compound in a yield of at least 70%, which is compatible with industrial applications.